The invention concerns image data processing. More particularly, the invention relates to an improved method and arrangement utilizing a discrete frequency transform data compressor wherein the compression characteristics can be modified in real time on a per document basis.
Financial documents, such as checks or drafts, usually contain a magnetic ink character recognition (MICR) code line which is typically read by a sorter and which automatically sorts these documents into a plurality of bins. Prior to sorting, in current practice, these sorted documents are presented to banking personnel who physically handle the documents and mechanically enter, in MICR form, the dollar amount associated therewith upon each document by use of specialized amount entry equipment. Additionally, these individuals physically correct incorrectly read MICR data associated with each of the sorted documents.
These prior techniques of utilizing banking personnel to process financial documents have proven relatively costly and inefficient, in that many of these documents have been lost or destroyed during their physical handling, and the speed associated with the processing of the documents is limited to that associated with the processing capabilities of the banking personnel and the mechanical amount entry equipment they use.
To address these problems, document imaging systems have been introduced to eliminate much of the previously required physical handling of the documents themselves. In such known systems, functions such as image data element normalization, background suppression, scaling and document height detection are performed as preprocessing functions prior to compression of the document image data for storage or transmission to a data decompressor.
Previous approaches for effecting background suppression in document image processing systems to yield higher contrast images for data entry operators are complex. Additionally, for imaging systems to be associated with archival storage systems, the original image data cannot be discarded in a suppression technique, but must be maintained for archival integrity in order to store the document images in a near-photographicquality form.
One class of known compression techniques involves transforming gray level pixel values of the original document image data into a spatial frequency domain via a transform type compressor. The Joint Photographic Experts Group of the International Standards Organization (ISO/JPEG) is currently developing an international standard compression technique and interchange format which may be used to compress continuous tone gray scale or color imagery. The compression standard under development by this group is commonly referred to as the ISO/JPEG (or JPEG) image compression algorithm, and it is based upon a transform process known as the Discrete Cosine Transform. The JPEG compression standard is outlined in JPEG Technical Specification, Revision No. 6, dated Jan. 14, 1991. This document draft is also known as JPEG-9-R6.
The Discrete Cosine Transform (DCT) image compression technique in its broadest aspects may be summarized as follows. Each document's image pixel data is partitioned into 8.times.8 sample blocks. Then, each 8.times.8 sample block is transformed mathematically using the DCT into a set of 64 values referred to as the DCT coefficients. Each of these 8.times.8 64 element coefficient arrays may be considered as a block of frequency coefficients in the DCT method. One of the block's coefficients is referred to as the DC coefficient, while the other 63 are referred to as the AC coefficients. Each DCT coefficient contains information concerning the spatial frequencies associated with that block of original image pixel samples. The DC coefficient is proportional to the average value of all the samples located in the block and represents the DC level (i.e. the zero spatial frequency) associated with that block of sample pixels. The AC coefficients indicate the degree to which certain spatial frequencies are present or absent in the sample 8.times.8 block being examined.
The DCT coefficients are next quantized using a predetermined and separate integer quantization factor for each coefficient in the block. The 64 integer quantization factors are stored in a 64 element array known as the quantization matrix. Quantization is performed by dividing each DCT coefficient by its corresponding quantization matrix element and rounding the result to the nearest integer value. Quantizing the DCT coefficients contributes significantly to the image compression process by discarding data that is considered not visually significant. The quantization matrix controls the tradeoff between image fidelity and image compressibility.
Finally, statistical characteristics of the quantized coefficients are used to encode the quantized coefficients present in the 8.times.8 sample block.
The instant invention is directed to a document image compression approach which (a) uses a JPEG image compression algorithm to compress full gray scale document imagery, (b) provides a method to optimize the JPEG standard for compressing images and (c) provides a method to monitor and modify document image compression in real-time on a per document basis.